1. Field of the Invention
This invention relates generally to voltage reference circuits, and more particularly to voltage reference circuits having very low noise specifications.
2. Description of the Related Art
One type of voltage reference circuit having a low or zero temperature coefficient (TC) is the bandgap voltage reference. The low TC is achieved by generating a voltage having a positive TC (PTAT) and summing it with a voltage having a negative TC (CTAT) to create a reference voltage with a first-order zero TC. One conventional method of generating a bandgap reference voltage is shown in FIG. 1. An amplifier 10 provides equal currents to bipolar junction transistors (BJTs) Q1 and Q2; however, the emitter areas of Q1 and Q2 are intentionally made different, such that the base-emitter voltages for the two transistors are different. This difference, ΔVBE, is a PTAT voltage which appears across resistor R2. It is summed with the base-emitter voltage (VBE) of Q1, which is a CTAT voltage, to generate reference voltage VREF, which is given by:VREF=VBE,Q1+VPTAT=TBE,Q1+K(VT ln(+VOS),where K=R1/R2, VT is the thermal voltage, N is the ratio of the emitter areas and VOS is the offset voltage of amplifier 10.
When so arranged, the noise vn,PTAT generated in the creation of the PTAT voltage is given by:vn,PTAT=√{square root over ((vn,amp2+vn,Q12+vn,Q22+vn,R22)K2+vn,R12)}
Another bandgap voltage reference approach, described in U.S. Pat. No. 8,228,052 to Marinca, is illustrated in FIG. 2. Explicit amplifiers are not used with this ΔVBE voltage generation method in favor of stacked, independent ΔVBE cells. Here, the output of the voltage reference is given by:VREF=ΔVBE1+ΔVBE2+ . . . +ΔVBEK+VBE The noise of each ΔVBE cell is uncorrelated with the others; thus, the noise contributions to the PTAT voltage, vn,PTAT, sum in an RMS fashion as given by:vn,PTAT=√{square root over (vn,ΔVBE12+vn,ΔVBE2+ . . . +vn,ΔVBEK2)}Though this approach generates less noise that the conventional approach shown in FIG. 1, the noise level may still be unacceptably high for certain implementations.